When fluid is conveyed through, for example, a conduit or pipe under pressure, a flow rate is usually controlled by a metering orifice and a suitable actuator such as an electromagnetic valve, etc. The actuator, based on a control signal applied thereto, controls the flow rate of fluid by changing the pressure applied to the fluid or a cross sectional area of the pipe. However, the actuator mechanically controls the flow rate so that it is unable to abruptly change the flow rate due to a relatively large transient time inherent in the mechanically operated actuator. Furthermore, in the case where the flow rate is controlled by "open" and "close" operations of the actuator, undesirable pulsating flow is caused. Therefore, the prior art has not been suitable for an accurate control of the flow rate.